Net::Pcap is a Perl binding to the LBL pcap(3) library and its Win32 counterpart, the WinPcap library. Pcap (packet capture) is a portable API to capture network packet: it allows applications to capture packets at link-layer, bypassing the normal protocol stack. It also provides features like kernel-level packet filtering and access to internal statistics.

Since Net::Pcap version 0.08, released in October 2005, the module modified the internal variable PL_signals to re-enable immediate signals delivery in Perl 5.8 and later within some XS functions (CPAN-RT #6320). However, it can create situations where the Perl interpreter is less stable and can crash (CPAN-RT #43308). Therefore, as of version 0.17, Net::Pcap no longer modifies PL_signals by itself, but provides facilities so the user has full control of how signals are delivered.

First, the pcap_perl_settings() function allows one to select how signals are handled:

:functions short names of the functions (without the "pcap_" prefix) for those which would not cause a clash with an already defined name. Namely, the following functions are not available in short form: open(), close(), next(), dump(), file(), fileno(). Using these short names is now discouraged, and may be removed in the future.

By default, this module exports the symbols from the :datalink and :pcap tags, and all the functions, with the same names as the C library.

The first form was introduced by Marco Carnut in Net::Pcap version 0.05 and kept intact in versions 0.06 and 0.07. The second form was introduced by Jean-Louis Morel for the Windows only, ActivePerl port of Net::Pcap, in versions 0.04.01 and 0.04.02.

The new syntax has been introduced for consistency with the rest of the Perl API and the C API of libpcap(3), where $err is always the last argument.

Determine the network number and netmask for the device specified in $dev. The function returns 0 on success and sets the $net and $mask parameters with values. On failure it returns -1 and the $err parameter is filled with an appropriate error message.

Returns a packet capture descriptor for looking at packets on the network. The $dev parameter specifies which network interface to capture packets from. The $snaplen and $promisc parameters specify the maximum number of bytes to capture from each packet, and whether to put the interface into promiscuous mode, respectively. The $to_ms parameter specifies a read timeout in milliseconds. The packet descriptor will be undefined if an error occurs, and the $err parameter will be set with an appropriate error message.

Return a packet capture descriptor to read from a previously created "savefile". The returned descriptor is undefined if there was an error and in this case the $err parameter will be filled. Savefiles are created using the pcap_dump_* commands.

Read $count packets from the packet capture descriptor $pcap and call the perl function &callback with an argument of $user_data. If $count is negative, then the function loops forever or until an error occurs. Returns 0 if $count is exhausted, -1 on error, and -2 if the loop terminated due to a call to pcap_breakloop() before any packets were processed.

The callback function is also passed packet header information and packet data like so:

sub process_packet {
my ($user_data, $header, $packet) = @_;
...
}

The header information is a reference to a hash containing the following fields.

len - the total length of the packet.

caplen - the actual captured length of the packet data. This corresponds to the snapshot length parameter passed to open_live().

Sets a flag that will force pcap_dispatch() or pcap_loop() to return rather than looping; they will return the number of packets that have been processed so far, or -2 if no packets have been processed so far.

This routine is safe to use inside a signal handler on UNIX or a console control handler on Windows, as it merely sets a flag that is checked within the loop.

Please see the section on pcap_breakloop() in pcap(3) for more information.

Return the next available packet on the interface associated with packet descriptor $pcap. Into the %header hash is stored the received packet header. If not packet is available, the return value and header is undefined.

Compile the filter string contained in $filter_str and store it in $filter. A description of the filter language can be found in the libpcap source code, or the manual page for tcpdump(8) . The filter is optimized if the $optimize variable is true. The netmask of the network device must be specified in the $netmask parameter. The function returns 0 if the compilation was successful, or -1 if there was a problem.

Set the non-blocking mode of a live capture descriptor, depending on the value of $mode (zero to activate and non-zero to deactivate). It has no effect on offline descriptors. If there is an error, it returns -1 and sets $err.

In non-blocking mode, an attempt to read from the capture descriptor with pcap_dispatch() will, if no packets are currently available to be read, return 0 immediately rather than blocking waiting for packets to arrive. pcap_loop() and pcap_next() will not work in non-blocking mode.

Open a savefile for writing and return a descriptor for doing so. If $filename is "-" data is written to standard output. On error, the return value is undefined and pcap_geterr() can be used to retrieve the error text.

Returns, on Unix, a file descriptor number for a file descriptor on which one can do a select() or poll() to wait for it to be possible to read packets without blocking, if such a descriptor exists, or -1, if no such descriptor exists. Some network devices opened with pcap_open_live() do not support select() or poll(), so -1 is returned for those devices. See pcap(3) for more details.

Accepts a set of strings (host name, port, ...), and stores the complete source string according to the new format (e.g. "rpcap://1.2.3.4/eth0") in $source.

This function is provided in order to help the user creating the source string according to the new format. An unique source string is used in order to make easy for old applications to use the remote facilities. Think about tcpdump(1), for example, which has only one way to specify the interface on which the capture has to be started. However, GUI-based programs can find more useful to specify hostname, port and interface name separately. In that case, they can use this function to create the source string before passing it to the pcap_open() function.

Returns 0 if everything is fine, -1 if some errors occurred. The string containing the complete source is returned in the $source variable.

Parse the source string and stores the pieces in which the source can be split in the corresponding variables.

This call is the other way round of pcap_createsrcstr(). It accepts a null-terminated string and it returns the parameters related to the source. This includes:

the type of the source (file, WinPcap on a remote adapter, WinPcap on local adapter), which is determined by the source prefix (PCAP_SRC_IF_STRING and so on);

the host on which the capture has to be started (only for remote captures);

the raw name of the source (file name, name of the remote adapter, name of the local adapter), without the source prefix. The string returned does not include the type of the source itself (i.e. the string returned does not include "file://" or "rpcap://" or such).

The user can omit some parameters in case it is not interested in them.

Returns 0 if everything is fine, -1 if some errors occurred. The requested values (host name, network port, type of the source) are returned into the proper variables passed by reference.

Open a generic source in order to capture / send (WinPcap only) traffic.

The pcap_open() replaces all the pcap_open_xxx() functions with a single call.

This function hides the differences between the different pcap_open_xxx() functions so that the programmer does not have to manage different opening function. In this way, the trueopen() function is decided according to the source type, which is included into the source string (in the form of source prefix).

Returns a pointer to a pcap descriptor which can be used as a parameter to the following calls (compile() and so on) and that specifies an opened WinPcap session. In case of problems, it returns undef and the $err variable keeps the error message.

Send a raw packet to the network. $pcap is the interface that will be used to send the packet, $packet contains the data of the packet to send (including the various protocol headers). The MAC CRC doesn't need to be included, because it is transparently calculated and added by the network interface driver. The return value is 0 if the packet is successfully sent, -1 otherwise.

Adds a packet at the end of the send queue pointed by $queue. The packet header %header has the same format as that passed to the loop() callback. $ackekt is a buffer with the data of the packet.

The %headerr header structure is the same used by WinPcap and libpcap to store the packets in a file, therefore sending a capture file is straightforward. "Raw packet" means that the sending application will have to include the protocol headers, since every packet is sent to the network as is. The CRC of the packets needs not to be calculated, because it will be transparently added by the network interface.

This function transmits the content of a queue to the wire. $pcapt is the interface on which the packets will be sent, $queue is to a send_queue containing the packets to send, $sync determines if the send operation must be synchronized: if it is non-zero, the packets are sent respecting the timestamps, otherwise they are sent as fast as possible.

The return value is the amount of bytes actually sent. If it is smaller than the size parameter, an error occurred during the send. The error can be caused by a driver/adapter problem or by an inconsistent/bogus send queue.

Please report any bugs or feature requests to bug-Net-Pcap@rt.cpan.org, or through the web interface at http://rt.cpan.org/Dist/Display.html?Queue=Net-Pcap. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

Currently known bugs:

the ps_recv field is not correctly set; see t/07-stats.t

pcap_file() seems to always returns undef for live connection and causes segmentation fault for dump files; see t/10-fileno.t

pcap_fileno() is documented to return -1 when called on save file, but seems to always return an actual file number. See t/10-fileno.t